Touch plate ground wire

Has anyone modified the way the magnet is attached to the wire? I grow weary of the tiny screw and nut being loose at the magnet. I was thinking about soldering the wire on. Of course with the least possible heat.

Ha! Mine just broke off and I never reattached it.

The CNCDepot has a correctly grounded chassis and as such you don’t need it anyways.

Now you are on to something! I’ve seen these touch plate permanently mounted on machines and no wire is attached to the spindle when the touch plate is used. So hook a brother up…

A little dab of blue locktite maybe?

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Have you tried to see if the GMT spindles are grounded properly?

The only way that core isn’t grounded is if they have ceramic bearings but even the S30C has a metal bearing on the top.

When I sold that GMT spindle on eBay I had to settle for $180. Never used and still nobody would buy it. Eventually I slapped it out there for a buck and 3 people bid on it. $180 was the selling price. Unopened, all the collets and everything AvidCNC sold me.


Well if I don’t attach the magnet wire to the spindle it will push down on the brass plate, so I guess the gmt is in the improperly grounded state… (frown)

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You can ground your sensor wire to the frame near the Z axis to eliminate the need for a magnet. I did it I with an m12 splitter and a field wireable connector.

I also moved my touch plate to the Z axis so I don’t need a long wire from the control box, but that’s not required.


It would be nice if all the spindles had an intentional path to the shaft from the chasis for this. My Hiteco 4hp spindle has anything from a few ohms to 20k ohms depending on how you move the shaft around. That would make me nervous about counting on it for the touchplate.

One of my wierd thoughts was “what if there was a way to have a tool holder that included electrical connections, that the ATC could hotplug when the tool inserted, for stuff like this.” Obviously you’d need to prevent the spindle from rotating.

At the very least, a spring-loaded arm or something that went from the ER portion of the toolholder (or from the pin you’re probing with) to press against some metal part of the spindle, would do the trick - as long as it could be kept out of the way and properly oriented.

I have that same spindle. You should be fine doing it like I did… did you check for continuity on the Z axis itself?

I checked from the body of the motor to the rest of the machine and have very good continuity (<1 ohm). However, to the output shaft it is very variable. I can make it a nice low resistance if I side load the probe force on the collet. So the spindle bearings are conductive if you load them. However, it is a pretty high force (at least a few pounds) and I wouldn’t trust mine to give me consistent touchplate results through the bearings with the amount of load it needs.

That’s odd… my reading is perfect and I have the same spindle. Mine is likely a lot newer and has less hours than yours though…

In the past on different machine builds I’ve never had connectivity issues when grounding through the spindle like that.

Interesting… your spindle isn’t worn out to the point of having some play in it is it?

I really hope not. I probably have 100-150 hrs on that spindle max. I don’t feel any play in it.

I haven’t run it for a couple of weeks. Now I’m curious to see what happens if I warm it up. I’ll try that this afternoon.

There is no question the bearings are worn in the spindle.

The bearing in a spindle are always preloaded so any movement of the shaft would be bearings at the end of their life.

Well, I have no measureable runout, and if I put a 2" long 0.5" shank bit in at push and pull hard, I can’t get it to move more than 0.0004" total swing, and that did not change at all between cold and after a warmup cycle. I don’t know what it was like when it was new because I didn’t measure it, but I woudl be surprised if a new one didn’t move that much with that much load.

BTW, the manual for the Hiteco says they are ceramic bearings, so I’m not sure I would expect them to be conductive. (it does’t say if both bearings are ceramic, or only the collet end).

However, that brings up a question I’m more concerned with (because I don’t really care if I can get rid of the ground magnet connection or not, I was just curious about the contact threw the bearings :-)).

We’ve had a few discussions about maintanence and lifetime of the axis drive gear and linear rails in here, but I don’t recall any discussion about the lifetime expectation of the spindle bearings are, and how much runout or side loaded slop would be considered “worn out”. I don’t see anything on the AVID site or in the spindle spec sheet.

I’d be interested if anyone knows what to expect, or if anyone has actually worn theirs out and how many hours they thought they had on it.


Well, I know that is how everyone tests their runout on their spindles but that isn’t how they do it at the manufacturer.

They will lay the spindle horizontal and hang a weight from the tip of the tool mount and then measure it that way. The vertical setting of the spindle will naturally give you a better reading. The weight in question will be the maximum side loading.

But! I thought you were talking about one of those awful GMT things. I am unfamiliar with your spindle so there can be other issues.

I am very suspicious that you can’t measure runout but you can see the resistance change. Grease is a natural insulator and the only connector between the shaft and the ground will be through the metal top bearing. I imagine the grease is not evenly surrounding the top bearing and thus your variance.

I would be interested in seeing if you go through warmup and then test to see if the grease in that metal bearing is just uneven after cooling.

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I originally had the 3 hp GMT but upgraded to the 4 hp Hiteco when they came out (because I like shiny new things, and wanted a little more HP for planing mesquite river tables).

So I just went out after it had cooled down and measured resistance and it was just as variable.

Since it seems that the top bearings might be steel and the bottom ceramic, and I have been side loading the collect end, I was curious so I pulled the fan cover off the top and pushed the top of the shaft around while measureing the resistance from the spindle housing to the collet.
This was very different. Even a very small push with a finger would get me a near 0 ohm path. So I think the bottom bearings are non conductive ceramic, the top are steel, and at least on mine the top tends to rest in the center with light to no contact when it is turned off. Probably a combination of the grease in there, and the seals centering the shaft, which woudl imply that at least that top bearing is not preloaded (or at least not any more).

I’m still curious though to know what the estimated life of these bearings are.

Yep, interesting. So you are getting a bit of wobble in there.

I can’t tell you any more than that so your best bet is ask the factory. You might just have a bad bearing. It might still be under warranty.

(Or I can be a complete idiot and nothing is wrong with it… that is always an option I fail to mention :rofl: )

Figured I’d chime in here:

In the past on several non Avid machines, and several spindles, and a few different controls I’ve done this same setup. It’s worked for me with an S30, HSD, HSD knockoffs, air drills, etc.

I’ve tested the accuracy of this on my setups and it’s been spot on. It was better than my metal touch off plate (which wasn’t the nice spring loaded Avid one, it was just a hunk of Al U Minium)

The name of the game here is that you want more than zero resistance but you don’t need a “perfect” signal. All the control needs to see is that we’ve gone infinite resistance to “not infinite”. So @jjneeb I’m certain your spindle is fine.

As you can see from my scattered list of tested spindles that Hiteco wasn’t on there. I’ve since found out that this is a supported workflow from Hiteco, and that a large number of their customers do this (even with ceramic bearings). As a test group of 1 I can say that it works great on my machine :slight_smile:

I did this on my machine because I was so used to how I did it on my other machines that I wanted to duplicate it. So far it’s worked well for me, and I’ve tested it for accuracy the same way I did in my previous setups.

This isn’t officially supported by us, this is just me tinkering and sharing the results. So… proceed at your own risk and hopefully you find this interesting.


You are correct, as long as the spindle impedance is still relatively low compared to the input impedance of the ESS input that is sensing that connection it will still work fine.

So of course you can’t really know for sure unless you make a spreadsheet…:slight_smile:

I chucked up a 1/2" downcut endmill (unfortunately I don’t have any precision dowels on hand) and did 10 touchplate runs with the magnet on the bit, 10 with the magnet on one of the bolts that holds the tramming plate on, and 5 more with the magnet stuck to my rotary unit’s chuck. I gave the spindle a spin with my fingers between each one. I had to rehome on the 8th run because the magnet was on a dirty part of the bit and I had to EMO the machine.

I recorded the machine coordinates after each run, normalized it to graph with better resolution, and plotted it.

There is more variation in the X and Y than I’ve seen on past tests, but this bit was a little harder to align the edge to X and Y directions, so I think this was my ability to align the tip perpendicular to the Axis. Wish I would have had a dowel. Z is very tight. Because Z was so repeatable, you can see that the Z re-home did shift a couple thousandths.

From the data, you can’t see any variation between where the ground wire is connected, even on my spindle that has 2k ohm worth of variation in the bearing resistance.

At this point I would call grounding through the chassis more reliable than the magnet because the magnet wire can break, you can forget to put it on, or you can set it on some tar on the bit (that has happend a couple times to me).

I have a couple lasers with the touchplate rod that aren’t grounded so I may keep my magnet and wire on the touchplate, but its nice to have the option and backup through the chassis as well.